The conventional Cassegrain monopulse, or simultaneous lobing, antenna system comprises a main reflector, a sub-reflector and a set of monopulse feeds arranged in the conjugate focal region of the sub-reflector. The set of monopulse feeds typically comprises four feed elements offset a small distance from each other along the boresight axis. When the antenna operates in a receiving mode, the outputs of the individual feed elements are processed so as to provide sum and difference channel signals. The sum channel signal corresponds to a narrow beam of radiation lying along the boresight axis of the antenna. The difference channel signal for four feeds corresponds to a pair of V-shaped beams of radiation that are orthogonal to one another and have a lone null orthogonal to the boresight axis. In the receiving mode, R.F. energy reflected from or generated by a target is collected by the main reflector of the monopulse antenna, and detected by the monopulse feeds at the conjugate focal region of the sub-reflector. Using conventional hybrid junction means, signals from four feeds are added and subtracted to provide information on the angular deviation of the target from the boresight axis of the antenna. Range can also be obtained if desired.
The details of the conventional monopulse system are well known and will not be discussed in depth herein. A rigorous discussion of monopulse is found in Rhodes, D.R., "Introduction to Monopulse", McGraw-Hill Book Company, Inc., 1959. Also, for brevity, since it is well known that antennas are reciprocal (have the same characteristics for transmitting as for receiving), the discussion of the antenna is made with respect to a receiving antenna with the understanding that similar principles apply for a transmitting antenna.
The field of view, or beamwidth, of an antenna relates to the size of the region subtended by the main beam of the antenna radiating pattern. For example, in the case of a narrow beamwidth, high gain monopulse antenna, the antenna is capable of very accurately tracking a target within a narrow field of view. However, using a narrow beamwidth antenna, it is difficult to initially sight a small target in a large search area, in a mode termed "acquisition".
One approach for effecting acquisition is to cause the narrow beam to scan the search area using scanning patterns such as zig-zag or conical. Another approach is to provide means for switching or zooming the antenna between narrow beamwidth and wide beamwidth operation. The antenna is first operated in a wide beamwidth mode to view a relatively large search region for the target. Once the antenna is pointed so that the target would be in its narrow field of view, acquisition has occurred and the antenna is switched, or zoomed, to its narrow beamwidth mode to take advantage of greater gain or efficiency. The narrow beamwidth mode is then maintained, tracking the target by conventional simultaneous lobing techniques.
The acquisition problem is particularly acute for narrow beamwidth antennas having large main reflector dishes of the type considered by the National Aeronautics and Space Administration for Tracking and Data Relay Satellites (TDRS) to relay to earth data collected from orbiting earth observation satellites or spacecraft. These antennas, operating at 15 Gigahertz, would have a main dish diameter of approximately 12.5 feet (3.8 meters) with a consequent narrow half power beamwidth of approximately 0.4.degree.. Initial pointing of the narrow beamwidth antenna of the data relay satellite toward an earth observation satellite or spacecraft would be difficult to achieve because of attitude instabilities of the data relay satellite and time variant radiation pattern offsets resulting from non-uniform solar heating of the antenna. Thus, a means for increasing the beamwidth of the antenna to effect acquisition is required.
Numerous techniques for switching or zooming the beamwidth of a monopulse antenna have been considered for TDRS. For example, in one prior technique, the antenna is defocused by axially shifting either a feed or a subreflector. This technique is undesirable for spaceborne equipment, such as TDRS, because moving parts are required. This increases the possibility of component failure, and increases the weight of the satellite by requiring additional fuel for its operation. Further, the antenna pattern amplitude and phase characteristics have a tendency to become distorted as the beamwidth is widened, and compensation apparatus substantially increases the complexity of the system. Also, the efficiency of the antenna, particularly in the wide beamwidth mode, is reduced.
In another prior technique, a polarization sensitive grating is placed in front of the main dish to serve as a smaller main dish for the wide beamwidth mode. This grating, although smaller than the dish, is sufficiently forward therefrom to intercept substantially all radiation emanating from a feed. The grating passes only one of vertically or horizontally polarized radiation toward the main dish and reflects the other. The radiation that is passed to the main dish provides a narrow beamwidth pattern; the radiation that is reflected from the grating provides a wide beamwidth pattern. Thus the antenna beamwidth is switchable by switching between vertical and horizontal feed polarization. However, the technique does not permit the use of circular polarization, which has both vertical and horizontal polarization components.
Another prior technique, for switching antenna beamwidth is disclosed in the patent to Schmidt U.S. Pat. No. 3,866,233, commonly assigned with the present invention, wherein a feed means selectively excites only the central region of a main dish reflector for wide beamwidth, or substantially the entire main dish reflector for narrow beamwidth. In one embodiment disclosed in that patent, a convex hyperbolic subreflector is provided with an outer annular region that is selectively translated along the main dish boresight axis to excite different areas of the main dish in response to excitation from a single feed located on the axis. In a second embodiment, an ellipsoidal subreflector is provided in view of the main dish reflector and has two foci on each of which is positioned a separate feed. One feed faces the main dish and excites its entire surface for narrow beamwidth operation; the other feed faces the subreflector and excites only a portion of the main dish reflector via the subreflector for wide beamwidth operation. While these switchable beamwidth antennas perform satisfactorily, they require movable parts or feed support structure redesign. These requirements are potentially impracticable in some spaceborne equipment. For many applications, there still exists a need for providing switchable beamwidth operation in a monopulse antenna that does not require any moving parts, complete redesign of the feed support structure, or substantially increased complexity.